The Marine captain being treated at Walter Reed Army Medical Center for a possible case of rabies has improved slightly, medical authorities said yesterday.

"He's not completely out of the woods, by any means," said Dr. Kenneth W. Bernard, an epidemiologist at the national Centers for Disease Control in Atlanta, who has been studying the case. "We still have no idea what is really going on . . . . We know he has a viral disease of the central nervous system, with absolutely no evidence of what it is."

The marine, who was admitted to Walter Reed on March 2 after three days of treatment at Fort Belvoir's DeWitt Army Hospital, has "improved muscle tone" and no longer requires continual use of a respirator, a Walter Reed spokesman said. He still has encephalitis, or inflammation of the brain, and his temperature fluctuates widely, the spokesman said.

Authorities, at the request of the marine's family, have declined to reveal his identity.

Since his hospitalization, he has drifted in and out of consciousness, but when awake has been able to speak and recognize people. That alertness and ability are two factors that lead doctors to believe he is suffering from rabies, rather than some other viral infection, Bernard said.

"Rabies typically affects the lower functions . . . and the peripheral nerves and has a tendency to spare the higher functions," Bernard said.

Although rabies antibodies have not been found in the marine's blood, Bernard said no antibodies were ever found in the blood of an Arizona man who died of rabies in 1981. An autopsy disclosed the antibodies in his brain.

In addition to rabies, the marine is also being tested for 12 polio-related viruses that have been recorded in Virginia in the last year. So far, the tests have been negative on a half-dozen of those viruses.